Narrowboats come in a range of lengths and styles. They are 7' wide and up to 70' long. Normally with engines, they can cruise most of the inland waterway system. Very popular as an 'all round' craft, but the narrow beam does limit space. The hulls of narrow boats are generally made of steel, with the cabins constructed usually of steel, but also of wood or fibreglass. Most narrow boats built in the last 20 years will be of all steel construction. The most common construction size of steel is called 10-6-4. This refers to 10mm Bottom, 6mm to gunwale level and 4mm for the rest.